The present invention relates to a circuit for compressing the dynamic range of the power of input signals to a transmitting signal amplifier and a receiving amplifier, and a power amplifying circuit using the dynamic range compressing circuit.
Recently multi-carrier radio communication schemes are widespread which permit high-speed transmission possible the use of plural narrow-band carriers. As compared with a single-carrier high-speed transmission, the multi-carrier high-speed transmission is less susceptible to fading or some other influences of changes in the propagation path, and hence is more robust against delay waves. Furthermore, the multi-carrier system possesses the advantages of simplifying radio circuitry and relaxing the requirements imposed on the radio circuit used.
The multi-carrier radio communication schemes, thus suitable for the high-speed transmission, have been practiced in fixed microwave communication systems and multi-channel access systems.
In recent years there has been proposed an OFDM (Orthogonal Frequency Division Multiplexing) radio communication scheme intended for high-speed transmission in the microwave band such as 5-GHz band on IEEE802. 11. In the field of broadcasting the application of the OFDM system to the next-generation digital television is now under study.
These multi-carrier radio communication schemes contain various features, but have such problems as an increase in out-of-band leakage power due to intermodulation distortion and the occurrence of intersymbol interference by nonlinearity of transmitters. The intermodulation distortion on transmitters occurs, for example, in a frequency converter or power amplifier. In particular, the influence of the nonlinearity of the power amplifier is great. In general, simultaneous amplification of multi-carrier will generate the intermodulation distortion if no output back-off is provided corresponding to PAPR (Peak-to-Average Power Ratio). Accordingly, the power amplifier for the multi-carrier transmission needs to be sufficiently high saturation output. Usually, the power amplifier becomes bulky and expensive with an increase in the saturation output because it involves an increase of the power supply and upsizing of radiation fins.
Heretofore, 4-carrier 16-QAM and 4-carrier 256-QAM systems have been put into use in the fixed microwave communication. The power amplifier in these systems combines individually amplified signals by a signal multiplexer having a quarter-wave line. The individual amplification of plural carriers is intended to avoid the problem of increased PAPR resulting from the multiplexing of the plural carriers.
Schemes that have been proposed so far to suppress PAPR in the multi-carrier transmission are: a scheme for setting initial phases of carriers (Shoichi NARAHASHI and Toshio NOJIMA, xe2x80x9cA New Phasing Scheme for Multitone Signal systems to Reduce Peak-to-Average-Power Ratio (PAPR),xe2x80x9d The Institute of Electronics, Information and Communication Engineers Transaction on B-II, Vol.J78-B-II, No. 11, pp.663-671, Nov., 1955); a scheme using a specific signal pattern that produces no peak (U.S. Pat. No. 5,381,449, xe2x80x9cPeak-to-average-power ratio reduction methodology for QAM communications system); a scheme using an error correcting code (T. A. Wilkinson and A. E. Jones, xe2x80x9cMinimisation of the peak to mean envelope power ratio of multicarrier transmission schemes by block coding,xe2x80x9d in Proc. 45th IEEE Vechi. Technol. Conf., pp.825-829, 1995); a scheme of multiplexing peak power suppressing signals (Shigeru TOMOSATO and Hiroshi SUZUKI, xe2x80x9cA Smooth Envelope Parallel Modulation/Demodulation Scheme,xe2x80x9d Technical Report of IEICE, RCS 95-77, Sep., 1995); and a scheme using orthonormal transformation (Japanese Patent Application Laid-Open Gazette No. 10-178411, corresponding U.S. patent application Ser. No. 08/948,090). In particular, there are known, as PAPR suppression schemes for OFDM, a scheme of clipping a multiplexed signal waveform (X. Li and L. J. Cimni, Jr., xe2x80x9cEffects of Clipping and Filtering on the Performance of OFDM,xe2x80x9d in Proc., 47th IEEE Vechi. Technol. Conf., pp.1634-1638, 1997) and a scheme of effecting transmitting output control according to peak power (Yoichi MATSUMOTO, Nobuaki MOCHIZUKI and Masahiro UMEHIRA, xe2x80x9cA Novel Peak Power Reduction Technique for Broadband Microcellular OFDM systems,xe2x80x9d Technical Report of IEICE, RCS 97-143, Oct., 1997).
The requirement for PAPR reduction in the multi-carrier radio communication is to prevent degradation of transmission performance without increasing the out-of-band power leakage. In terms of this requirement, the clipping of a multiplexed signal waveform causes an increase in the out-of-band power leakage. The control of the transmitting output according to peak power encounters difficulty in maintaining channel quality. The initial phase setting scheme is difficult to apply to a modulated wave of ever-changing phase. The utilization of orthonormal transformation is defective in that phase fluctuations of carriers degrade the inter-carrier orthogonality, resulting in the development of peak power. The use of an error correcting code and the multiplexing of peak power suppressing signals both involve enlargement of the transmission band. Thus, the conventional PAPR reduction schemes have such problems as the enlargement of the transmission band, difficulty in the application to modulated waves, an increase in the out-of-band distortion and difficulty in maintaining channel quality.
And, peak power reduction schemes applicable to modulated waves all involve signal processing of suppressing the peak power at the transmitting side and signal processing of reconstructing the received signal at the receiving side. To allow ease in the fabrication of a peak power suppressing circuit and achieve high-efficiency amplification that permits reduction in the size, weight and power consumption of the power amplifier, it is desirable to use a peak power suppression scheme that can be carried out within only the transmitting side as much as possible. In this respect, the initial phase setting scheme and a scheme using a PAPR reduction signal point are effective, but the former is difficult to apply to modulated waves and the latter is limited in the application to transmitting signal sequences.
It is therefore an object of the present invention is to provide an input signal power dynamic range compressing circuit that develops no out-of-band power leakage of the input signal thereto and permits effective compression of the signal power dynamic range irrespective of phase variations of the input signal, and a power amplifying circuit using the dynamic range compressing circuit.
The signal power dynamic range compressing circuit according to the present invention comprises:
a directional coupler for dividing an input signal into two signals:
a linear signal transfer path over which to linearly transfer the one of the two divided input signals;
a compressing signal generating path for generating a compressing signal that contains a component opposite in phase to the other distributed input signal; and
a power combiner for power-combining output signals from the linear signal transfer path and the compressing signal generating path.
A power amplifier is connected to the output of the signal power dynamic range compressing circuit to form a power amplifying circuit.